In various applications, having the capability to electronically steer an antenna in two directions may be desirable. As used herein, “steering” an antenna may include directing the primary gain lobe, or main lobe, of the radiation pattern of the antenna in a particular direction. Electronically steering an antenna means steering the antenna using electronic, rather than mechanical, means. Steering an antenna with respect to two dimensions may be referred to as two-dimensional steering.
Currently, two-dimensional steering is typically provided by phased array antennas. However, currently available phased array antennas have electronic configurations that are more complex and/or more costly than desired. Consequently, having some other type of antenna that can be electronically steered in two dimensions and that is low-cost relative to a phased array antenna may be desirable.
Artificial impedance surface antennas (AISAs) may be less expensive than phased array antennas. An artificial impedance surface antenna may be implemented by launching a surface wave across an artificial impedance surface (AIS) having an impedance that is spatially modulated across the artificial impedance surface according to a function that matches the phase fronts between the surface wave on the artificial impedance surface and the desired far-field radiation pattern. The basic principle of an artificial impedance surface antenna operation is to use the grid momentum of the modulated artificial impedance surface to match the wave vectors of an excited surface wave front to a desired plane wave.
Some low-cost artificial impedance surface antennas may only be capable of being electronically steered in one dimension. In some cases, mechanical steering may be used to steer a one-dimensional artificial impedance surface antenna in a second dimension. However, mechanical steering may be undesirable in certain applications.
A two-dimensional electronically-steerable artificial impedance surface antenna has been described in prior art. However, this type of antenna is more expensive and electronically complex than desired. For example, electronically steering this type of antenna in two dimensions may require a complex network of voltage control for a two-dimensional array of impedance elements. This network is used to create an arbitrary impedance pattern that can produce beam steering in any direction.
In one illustrative example, a two-dimensional artificial impedance surface antenna may be implemented as a grid of metallic patches on a dielectric substrate. Each metallic path may be referred to as an impedance element. The surface wave impedance of the artificial impedance surface may be locally controlled at each position on the artificial impedance surface by applying a variable voltage to voltage-variable varactors connected between each of the patches. A varactor is a semiconductor element diode that has a capacitance dependent on the voltage applied to this diode.
The surface wave impedance of the artificial impedance surface can be tuned with capacitive loads inserted between the patches. Each patch is electrically connected to neighboring patches on all four sides with voltage-variable varactor capacitors. The voltage is applied to the varactors through electrical vias connected to each patch. An electrical via may be an electrical connection that goes through the plane of one or more adjacent layers in an electronic circuit.
One portion of the patches may be electrically connected to the ground plane with vias that run from the center of each patch down through the dielectric substrate. The rest of the patches may be electrically connected to voltage sources that run through the dielectric substrate, and through holes in the ground plane to the voltage sources.
Computer control allows any desired impedance pattern to be applied to the artificial impedance surface within the limits of the varactor tunability and the limitations of the surface wave properties of the artificial impedance surface. One of the limitations of this method is that the vias can severely reduce the operational bandwidth of the artificial impedance surface because the vias also impart an inductance to the artificial impedance surface that shifts the surface wave bandgap to a lower frequency. As the varactors are tuned to higher capacitance, the artificial impedance surface inductance is increased, which may further reduce the surface wave bandgap frequency. The net result of the surface wave bandgap is that it does not allow the artificial impedance surface to be used above the bandgap frequency. Further, the surface wave bandgap also limits the range of surface wave impedance to that which the artificial impedance surface can be tuned.
Consequently, an artificial impedance surface antenna that can be electronically steered in two dimensions and that is less expensive and less complex than some currently available two-dimensional artificial impedance surface antennas, such as the one described above, may be desirable in certain applications. Therefore, it would be desirable to have a method and apparatus that take into account at least some of the issues discussed above, as well as other possible issues.